The application is related to modem protection circuits.
Advances in portable computing have enhanced the mobile work environment allowing persons to work and communicate with home or office from nearly any location. Hotels and other businesses catering to the mobile computer user seek ways to lure in the business by perhaps installing the latest in telecommunications systems for the business traveller. For example, some hotels are now providing digital Private Branch exchange (xe2x80x9cPBXxe2x80x9d) telephone lines into hotel rooms instead of the traditional analog telephone systems. PBX systems provide a telephone system with many more enhancements (e.g. messaging and call queuing) than an analog system while reducing the number of cables required to handle the communications.
All standard analog telephone systems provide a direct-current signal to each phone. To alert the end-user to an incoming call, a ring current is sent over the wires to energize, typically, an audible device (e.g. ringer for a bell). When the handset is picked up, a switch closes in the telephone and a current (called the tip current) flows over the wires. Equipment at the central office detects the current, and listens for the touch tones entered by the person dialing the number. The telephone company typically places approximately 48 volts across the tip and ring leads, but the current is limited by a resistance of 400 to 1750 ohms placed in series with the 48-volt source. Therefore, the resulting line current is usually less than. approximately 120 mA and, in most cases, the telephone company limits the current, to 60 mA or less.
Digital phone systems work differently. Like a LAN, a PBX communicates in digital pulses with the desk set. Because a PBX provides the power to run the microcontroller and other features in the desk set, more current is typically available on the phone lines. In a PBX digital system, a DC voltage is applied across the tip and ring leadsxe2x80x94typically in the range of 12 to 90 volts. However, the current from the supply is not limited and may typically exceed 90 mA.
In some ways, modems act just like traditional telephones: an internal switch connects tip and ring together. This switch is typically an electromechanical relay in most modems. PC Card modems, however, are much smaller and cannot accommodate the electromechanical relay, so solid-state relays are used. The relay is designed to dissipate heat from a 60-mA current and to withstand the higher AC voltages from a ring signal.
Digital systems typically use higher voltages and currents which are destructive to the analog modem. Most PC Card modems cannot dissipate the additional heat caused by the higher currents used in digital telephone systems. Consequently, the part of the modem circuitry that connects to the phone line is rendered inoperable, leaving the rest of the modem operating normally. This presents a problem to a user with a portable computer, for example, which typically includes the more common analog modem. The analog modem can only withstand analog line currents. This would not normally be of concern, except that there is no visible distinction between a digital telephone wall plug and an analog plug.
The way in which modems connect to the public telephone system has been largely standardized. This standardization is due to the need for telephonic systems to be compatible and interchangeable. The standard connector used in the United States is an RJ-11, 6-pin modular plug or receptacle. A typical wall telephone jack is an RJ-11 receptacle designed for interfacing to telephone or modem systems. Unfortunately for the analog modem user, a PBX digital system may also employ an RJ-11 receptacle in a wall jack, with no discernable difference. An unwary user would plug the analog modem into the digital jack and very quickly blow out a fuse in the modem, leaving it inoperable.
Common approaches use passive recoverable fuses in the analog modem front-end to open the circuit. Other more costly techniques may use active devices to control the current. Two considerations which must be addressed are the use of passive or active protection devices. An analog modem not constructed to withstand the high current associated with digital systems will not hold up long enough for a passive recoverable fuse-device to act. On the other hand, it is preferable to have a passive device for current control to minimize distortion on modem signal when operating under normal conditions.
Several modem manufacturers are integrating safeguards that prevent the modem fuse from being blown. For example, Hayes Microcomputer Products installs high-current resistors in PC Card modems to provide a xe2x80x9ccomfort factorxe2x80x9d for users. For another example, Megahertz uses a C40 chipset which includes a Digital Line Guard that disconnects the modem when the current exceeds 125 mA.
The application discloses an architecture for protecting telecom units against overcurrents. The architecture is particularly advantageous for preventing the destruction of an analog modem which has been plugged into a digital telephone system which uses current and voltages destructive to the modem. A main feature of this protective circuit is that it appears purely resistive under normal operating conditions. Therefore, the voltage-current characteristics are linear which results in a flat response across the voice band frequencies. The active devices provide a shunt path for overcurrents, but are not connected in the series path. Active devices also provide a fast response to overcurrent conditions. Preferably, the circuit also incorporates a resettable fuse as last-resort protection against substantial overcurrents not protected against by the active device circuit.
The invention has the principal advantage of using passive-device features which minimize noise and distortion of the modem signals under normal operating conditions.